The present invention relates to an automatic equalization apparatus for a recording system or a transmission system, and in particular to an automatic equalization method, and apparatus attaining high-speed, high-precision operation with simple configuration.
In order to prevent occurrence of intersymbol interference in a digital information train having a period T, its impulse response h(t) must, in general, satisfy the so-called Nyquist condition with the Nyquist frequency equivalent to 1/2T. That is to say, the following relations must be satisfied at time nT. ##EQU1##
An automatic equalizer automatically sets tap coefficients of a transversal filter so that the impulse response reproduced from a VTR, a disk or the like may satisfy equation (1). A so-called zero forcing algorithm is included in basic algorithms of automatic equalization as described in, for example, U.S. Pat. No. 3,414,845 issued on Dec. 3, 1968 to R. W. Lucky. An example of an apparatus based upon this scheme is shown in FIG. 2. By referring to FIG. 2, operation of this algorithm will now be described briefly and at the same time problems of this scheme will be made clear.
As shown in FIG. 2, a transversal filter typically comprises delay lines 1 and 2, gain adjusting circuits 3, 4 and 5, and an adder 6. It is now assumed that each gain adjusting circuit has a coefficient Cj. Assuming now that the impulse response from the information source to the equalizer output is h(t), the sum D of absolute values of intersymbol interference obtained after equalization is given by the following equation. ##EQU2##
In the zero forcing algorithm, the gain of the transversal filter is so controlled that the value of D may be minimized. Assuming now that the pulse train supplied from the information source has a value a.sub.k (where a.sub.k is a binary valued signal comprising "1" or "0") at time kT, the output of the adder 6 at time kT is given by the following equation. ##EQU3##
A signal e.sub.k corresponding to an equalization error is given by the following equation. EQU e.sub.k =y.sub.k -a.sub.k ' (4)
Character a.sub.k ' represents a value obtained by identifying and reproducing y.sub.k in a comparator 7 of FIG. 2 and coincides with a.sub.k in the absence of a code error. Character e.sub.k denotes an output of a comparator 8. By using the above described a.sub.k ' and e.sub.k, evaluation function H.sub.j of equalization error is given by ##EQU4## where m is a value depending upon the signal-to-noise ratio (SN ratio) and is typically in a range 10.sup.3 &lt;m&lt;10.sup.4.
The value of H.sub.j is derived by using a computer 10 shown in FIG. 2. By increasing the coefficient C.sub.j of the gain adjusting circuit by a minimum amount .DELTA. when H.sub.j is positive and by decreasing the coefficient C.sub.j by the minimum amount .DELTA. when H.sub.j is negative, the intersymbol interference D represented by equation (2) is reduced. If input data comprises a train in which "1" or "0" appears randomly, automatic equalization is attained by the zero forcing algorithm heretofore described.
The above described equalizer basically has two problems described below.
(1) In the configuration shown in FIG. 2, the main line signal which becomes the reference of H.sub.j passes through the gain adjusting circuit 4. Therefore, the gain adjusting circuit must be high in precision. Especially in a reproduced signal of a digital VTR, an amplitude variation having a high frequency component caused by defective contact between the tape and the head is incurred. For such a signal, gain adjusting circuits which are rapid in response speed become necessary. Further, since high-speed pulses of 100 Mbps or more are recorded onto/reproduced from the digital VTR, gain adjusting circuits having wide bandwidth become necessary. It is extremely difficult to realize gain adjusting circuits satisfying all of the conditions heretofore described.
(2) In addition, calculation of equation (5) must be executed at a speed of 100 Mbps in the above described equalizer configuration. Calculation of equation (5) is performed with respect to a series of m pulses depending upon the signal-to-noise ratio. Since dropouts often occur in the recording/reproducing system, however, a serious error is caused in the calculation result if pulses are missed consecutively.